A phased array is a group of radio frequency antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. In typical embodiments, they incorporate electronic, phase shifters that provide a differential delay or phase shift to adjacent radiating elements to tilt the radiated phase front and thereby produce far-field beams in different directions depending on the differential phase shifts applied to the individual elements.
A number of embodiments of delay lines and antenna elements can be arranged in an RF antenna assembly. The antenna assembly may include an array of the antenna elements. Such arrays of the antenna elements may, in certain embodiments, be spatially arranged in either non-uniform or uniform pattern to provide the desired antenna assembly characteristics. The configuration of the arrays of the antenna elements may affect the shape, strength, operation, and other characteristics of the waveform received or transmitted by the antenna assembly.
The antenna elements may be configured to either generate or receive RF signal. The physical structure of the element for signal generation and reception is similar, and typically a single element is used for both functions. A phase shifter/true time delay (PS/TTD) device is a crucial part of the antenna element providing a differential delay or phase shift to adjacent elements to tilt the radiated/received phase front.
The active phased array antenna architecture is the most applicable to the use of the PS/TTD device. A schematic of one of the embodiments of an active phased array antenna unit is shown in FIG. 1. The antenna element is connected to a circulator, which is used to separate the high power transmit path and the low power receive path, providing the required isolation. The receive path includes a limiter to avoid damage from a high input level, followed by a low noise amplifier (LNA) used to bring the received signal up to the required power level. The output of the LNA passes through a transmit/receive switch, and then through the phase shifter/true time delay (PS/TTD) device, which provides the correct phasing for that element before the output is summed with that from all other elements. The PS/TTD provides the correct phase shifting of each antenna element at all frequencies. The overall phased array antenna output power is a coherent addition of the signals from each of the antenna elements. A large number of elements provide a large total power for the system.
The tunable delay application is not limited to active phased array antennas. Alternatively, PS/TTDs can be implemented in passive phased array systems, where the power is shared passively between many antenna elements, each having its own PS/TTD device.
Photonics technologies offer significant advantages over RF and microwave electronics, which can be exploited in phased array systems. Optics offer tremendous inherent bandwidth for use in optical processing and communicating systems, due to the very high carrier frequencies (e.g. 200 THz) compared to the microwave signals (10 s GHz) upon which they operate. Photonic technologies offer much lower cost if efficiently integrated. Photonic devices are inherently small due to the short wavelength at which they operate (around 1 micron) compared to the cm, and min wavelengths of microwave, integrated circuits in phased array systems. Photonic integration provides a path to massive parallelism, providing additional reductions in size and weight, together with the promise of much lower overall system cost.
This invention relates to optical delay lines based on resonator structures, often reported in the literature as microresonator structures when describing resonators of small size. One of the most promising delay line designs is a ‘side-coupled integrated spaced sequence of resonators’ (SCISSOR) shown in FIG. 2 (a). SCISSOR structures are by definition all-pass filters with light propagating in only one direction, and thus they have zero reflection. U.S. Pat. No. 7,058,258 discloses an implementation of the side—coupled sequence of resonators for tunable dispersion compensation. It provides different group delays at different frequencies of the optical signal. The present invention addresses an opposite goal—to achieve exactly the same group delay over as wide range of frequencies as possible.
Another configuration (FIG. 2 (b)) of the side-coupled sequence of resonators was presented in U.S. Pat. No. 7,162,120, where the resonators are coupled to the opposite sides of the core waveguide. This configuration was designed only for the device compactness; there is no difference between the resonators on the both sides of the waveguide.
A multitude of phased array systems are used in many applications, varying from large surveillance systems to weapons guidance systems to guided missiles, plus many civil applications including weather monitoring radar systems, radio-astronomy and topography.
There is a need to provide more reliable and efficient devices for tunable delays to control phased array antennas. The best approach is in implementation of an optical device to provide extended bandwidth, cost reduction and compactness. As it was shown in our prior invention described in U.S. patent application Ser. No. 12/205,368, the tunability may be achieved either by the thermo-optical effect or by the quantum confined Stark effect or by carrier injection.
One of the key issues with using thermal tuning of a device, is that there are usually multiple time constants involved in the tuning, i.e. although the goal is to achieve fast (e.g. microsecond) tuning times, which may be achieved by the device structure itself, when it is done thermally there are typically much longer time constants also involved, due to the thermal mass of the whole device, the submount, the heatsink it is placed on, the cooling circuit that keeps the temperature constant etc. These much slower time constants, which can be as long as many seconds, cause changes in the device output on these longer timescales, which reduces the device performance, and in some applications makes the performance unacceptable.
There is a need to provide an improved tuning mechanism to achieve faster and more reliable performance.
A key issue with current manufacturing of resonators is the lack of uniformity of resonator response, in particular the lack of uniformity of resonator resonance frequency for identically designed resonators on the same device, additionally also the lack of uniformity of other performance parameters such as the amplitude of the response e.g. loss, delay, phase change and dispersion, and also the tuning performance of the response. An example of this non-uniformity in resonance frequency is shown in FIG. 2 (c), which is the measured transmission spectrum for a device with 10 identical resonators on it. Each individual resonator has a resonance dip close to 1555 nm, with a width of ˜0.3 nm and a free-spectral range of ˜9.5 nm. The combination of the 10 resonator responses provides a broad dip with a width of ˜1.2 nm, 4× the width of an individual resonator dip, with visible signs of individual resonator dips. In order to use this device in a tunable delay such as the current invention, there is a need for each of the resonators to be aligned with each other, at least for the zero detuning setting of the delay. Because of the resonance frequency non-uniformity, this requires individual control of each of the resonators, plus a measurement of each resonator to find its resonance frequency relative to the design, so that the resonator can be tuned to the design frequency. This adds significant complication to the use of this device in a practical application.
There is a need to provide a tunable delay with a broader bandwidth in order to support optical signals with broader bandwidth.